Science questions not worth a thread I: I'm a moron!

[El_Machinae]

FOCAL! Using gravity lensing to get an absolutely ginormous telescope.

(I really want it happen someday)

 

[warpus]

Michkov and uppi, thanks a lot for your input! That's exactly what I was after I think.

This is for a real-world project, not sure where it's going to go, but before I start working on it I need to figure out some numbers to see if my story would be feasible.

Nope, I'm not flying to another solar system, just working on a story

 

[Lohrenswald]

In what situation would you use speed as opposed to velocity?

 

[Arakhor]

As I recall, speed just indicates how fast something is going, whereas velocity requires a direction as well. It's the difference between scalar and vector quantities.

 

[Lohrenswald]

yes

the question is when you would use speed and not velocity

 

[Arakhor]

In general daily use, and perhaps when only talking about distance over time? If you're not talking in a physics context, everyone knows what 'speed' means, after all.

 

[Lohrenswald]

in general daily usage there is no distinction

also I'm talking about use in science, hence the choice of thread to ask the question in

 

[Cutlass]

Hydrogen peroxide at common household use levels, does that chemically break down and just be water? And how long does it take to do that?

 

[Birdjaguar]

I have a physics question and I'm not sure how to get started to do the math. If you give me the equations I can probably do it myself, but I need some help getting started.

Assumptions:

- There is a spaceship flying from point A to point B, the distance between them is d. It is assumed that this distance is light years, possibly between solar systems, but not necessarily.
- It accelerates at a constant rate for exactly half of the trip and decelerates for the second half at the exact same rate.
- The ship has a super amazing shield on the front and back that is able to absorb anything that can get in the way that can't be planned around ahead of time. So basically we don't have to worry about this in this scenario.
- Questions of propulsion type feasibility wrt acceleration (a) value can be ignored as well, unless the acceleration of a ship at the rate runs into any problems with any physical laws in some way. For the last question assume that the only limit is physics.
- The mass of the spaceship doesn't matter, but I realize that it plays a part in the equations. Assume that it's a variable that I can plug in as well. If a value has to be assumed for the explanation, assume that it weighs 10 times as much as a Dragon capsule so 40,000 kg or so.
- If there are people on-board you can assume that they will be safe no matter what the acceleration is due to future technology 58 shielding them from the effects

Questions:

1. How do I figure out how long the trip will take for values of distance d and acceleration a?

a. from the point of view of observers in solar systems A or B
b. from the point of view of observers on the moving ship

2. What maximum acceleration value a would be realistic given our current understanding of physics, if you assume an unlimited budget for a project to make it happen? If the only limit is physics, given what we know today, how fast can we make something accelerate? I looked here and 3 600 000 m/s² is double of a handgun and gives you a link to ultracentrifuges, which I think are tiny things, but either way that ball park seems like a good starting point maybe? A jellyfish stinger can accelerate at 53 000 000 m/s², is that a reasonable number? Or are these crazy numbers?

3. Are there any limits in the laws of physics for how fast you can accelerate something? Assuming that things getting in the way are not a problem.

The largest somewhat realistic acceleration for a spacecraft I have heard about were 200 000 m/s^2 for a tiny spaceship with a huge solar sail that is being pushed by lasers. But that only works as long as you are in the range of the lasers. The trick to fast space-travel is not a large one-time acceleration value, but to be able to maintain that acceleration. If you could maintain just 10 m/s^2 forever, you could cross the whole known universe in 100 years of proper time of the spacecraft. In earth's time you are obviously limited by the speed of light, so it would take tens of billion years until that spacecraft would be back (to an earth that might be non-existent by then). The Wikipedia article on proper acceleration has a nice graph that shows what would be possible with 10 m/s^2 (it also contains the necessary formulas):
https://en.wikipedia.org/wiki/Proper_acceleration



I do not think there are fundamental limits to (proper) acceleration. If you apply a very large force to a very tiny mass, you can get huge accelerations. More relevant would be practical limits (at which point would the force just destroy the vessel).



That is one interpretation of the Uncertainty Principle. Another is, that it limits to what extent reality is defined. So we cannot know what "really" is going on, because there is no "really" below that level. And so far, we do not know which of these interpretations is true.

Michkov and uppi, thanks a lot for your input! That's exactly what I was after I think.

This is for a real-world project, not sure where it's going to go, but before I start working on it I need to figure out some numbers to see if my story would be feasible.

Nope, I'm not flying to another solar system, just working on a story

Here is a great story based what you are asking. Awkright.

 

[Michkov]

Hydrogen peroxide at common household use levels, does that chemically break down and just be water? And how long does it take to do that?

Yes, over time it will decompose into water. The rate is dependant on multiple factors see such as temperature, concentration, etc, see link for more details

https://en.wikipedia.org/wiki/Hydrogen_peroxide#Decomposition

 

[Cutlass]

Yes, over time it will decompose into water. The rate is dependant on multiple factors see such as temperature, concentration, etc, see link for more details

https://en.wikipedia.org/wiki/Hydrogen_peroxide#Decomposition

I can't do the math. Can I assume a 10 year old quart bottle is just water and useless as a disinfectant?

 

[Michkov]

Check if there is an expiration date on the container to be sure but it should have lost most of the H2O2 by now. Especially if opened

 

[Cutlass]

The only thing which might be a date I can see says 0303 I'll just assume it's a bottle of water at this point.

 

[uppi]

In what situation would you use speed as opposed to velocity?

You would use speed if velocity is not relevant for your problem, you do not know enough to calculate the velocity, or if speed is the quantity you are interested in. Basically, every time the velocity/speed is squared in an equation.

For example, to calculate the kinetic energy of an object, you only need its speed. Conversely, if you only have the kinetic energy you can only calculate the speed, not the velocity.

Especially for mean values, speed can be much more useful than velocity. If you have a thermal gas at rest, the mean velocity of the constituting particles will be zero. However, the mean speed is not zero and can be immediately converted into a temperature for that gas. Therefore scientists tend to use the Maxwell-Boltzmann distribution (which is a speed distribution) to describe a gas.

The distinction between speed and velocity is a feature of the English language, anyway. The German language does not differentiate between those, with no serious consequences for physics.

 

[Atticus]

How does the temperature as a movement of molecules transform into the sensation of warmth? How does human sense that something is hot?

What is the point of the story of the apple and sir Isaac Newton? How could he have come up with his theory of gravity by observing it?

Is the usual practice in the physics papers to have one standard deviation as an error? That's what students are taught to do pretty much everywhere, but that corresponds to the confidence interval of 68 % (or does it not?), which sounds awfully sloppy to me.

 

[Arakhor]

We sense heat through our powers of thermoception. I do recall that the reason a seat feels warm after someone has been sitting on it is because the molecules in the seat reached thermal equilibrium with the molecules in the person's bottom, which is generally a different molecular state to the surrounding air (and to the second person's bottom).

 

[uppi]

Is the usual practice in the physics papers to have one standard deviation as an error? That's what students are taught to do pretty much everywhere, but that corresponds to the confidence interval of 68 % (or does it not?), which sounds awfully sloppy to me.

It is usual practice to give the standard error for all relevant numbers. If the readers know what it is (and it is good practice to state exactly what you error is), it is not sloppy. What kind of confidence you want of a number depends on the application and is kind of arbitrary. For a quick consistency check, two numbers matching within one standard error (or just above that) is usually good enough. If you want to show the existence of a new particle, you better be 5 sigmas away from the null hypothesis. With the standard error, you can quickly estimate your confidence in a number by multiplying the standard error with the factor you want. Giving the error in a standardized way has the advantage of simple comparisons between numbers. If everybody would make up their own way of giving errors (and there are lots of ways you could possibly define the error), you would always have to convert between different errors for any comparison (and you might not even have enough information to do that).

Note that converting between the standard deviation and the confidence interval (like 1 sigma = 68%) is only straightforward if you can assume your values to have a Gaussian distribution. Most of the time that is not that far from the truth, but especially if you have low count rates, your actual distribution can be quite different. In that case you can still give the error as the standard deviation, but claiming that this is the 68% confidence interval would be wrong! In that case the most stringent approach would be to give the probability density function for your measured value. However, this function is hard to calculate, would take a lot of space, and usually nobody cares that much. So people tend to give the standard error, even if the distribution is obviously non-Gaussian (e.g., error bars going above 1 for a value that has to be less than 1 by definition).

Most of the time, your distribution can be safely approximated by a Gaussian within one standard deviation, but the tails (the values far away from the estimated value) might have a very different distribution. So equating one standard deviation with a 68% confidence interval might be a fair approximation, but 5 sigma might be something very different than the 99.9999% interval of a Gaussian distribution. Estimating the tails of the distribution is a difficult task, because it involves very rare events by definition. Therefore, giving one standard deviation is more correct than giving some very large confidence interval. As scientists are afraid of stating something wrong, they go with the former.

Long story short: Use the standard error, unless there is a very good reason not to (and in that case, you need a paragraph to explain that).

 

[Samson]

We sense heat through our powers of thermoception. I do recall that the reason a seat feels warm after someone has been sitting on it is because the molecules in the seat reached thermal equilibrium with the molecules in the person's bottom, which is generally a different molecular state to the surrounding air (and to the second person's bottom).

Surely this is like saying "we hear by hearing". I am interested in the answer of how thermoreceptors work, but all I can find on the internet is "The details of how temperature receptors work is still being investigated." If anyone knows any more it would be great to hear.

 

[Arakhor]

Well, yes, it's not that helpful an answer, but you've identified that no one really knows and it does at least provide Atticus with the name of the sense for further study.

 

[Samson]

Well, yes, it's not that helpful an answer, but you've identified that no one really knows and it does at least provide Atticus with the name of the sense for further study.

It did, and to me too. Sorry if I came across derogatory, and I can see that it sounded that way.